Electromagnetic water current meters are well known in the art, see my previous U.S. Pat. Nos. 3,759,097 and 3,855,858, for examples of this apparatus. The latter-mentioned patent describes the use of electromagnetic water current meters for monitoring flow velocity in open bodies of water, i.e., rivers and oceans. In such an application, the transducer is supported from above, such as by being attached to a cable supported on land, by a boat, or by a buoy. In one configuration, the cable may serve to connect the electrodes in the transducer to signal processing apparatus which would normally be located on or above the surface of the body of water. In another application, however, the signal processing circuitry is included within the transducer housing, per se, and is thus submerged. In the latter application, conventionally, the housing would include a magnetic tape recorder or other type of data memory for storage of appropriate signals definitive of flow velocity at predetermined periods of time.
Whether the transducer is connected to surface mounted signal processing apparatus, or is self-contained, the voltages induced on the several electrodes can be employed to determine flow velocity with respect to the orientation of the transducer. If the transducer is not rigidly mounted, but instead can freely rotate, this data may not be completely meaningful. That is, measuring components of flow velocity with regard to an unknown coordinate system may not produce meaningful information. Rather, flow velocity should be determined with regard to some known coordinate system. Chambelle et al, in U.S. Pat. No. 3,641,816, discloses a "Device for Metering a Vector Particularly applicable to the Metering of a Fluid Flow Velocity". In short, that patent discloses an electromagnetic water current meter which is associated with a magnetometer for determining orientation of the meter with regard to the earth's magnetic field so that the components of flow velocity sensed by the transducer can be related to a fixed coordinate system, i.e., the conventional geomagnetic coordinate system used to define locations on the earth's surface with regard to the North magnetic pole. While there is no reason to believe that the device disclosed in the aforementioned patent does not operate quite well in its intended environment, there are limitations to the application of this structure.
The Chambelle apparatus includes a magnetic compass which determines direction by sensing the horizontal component of the earth's magnetic field. At the lower latitudes this is perfectly satisfactory. However, at high latitudes -- near the north and south magnetic poles -- the earth's magnetic field is largely vertical, having very little horizontal component, and large errors occur when one tries to use a magnetic compass to determine direction. In these areas, the patented apparatus might not produce useful information.
A further limitation on the employment of the apparatus disclosed in the '816 patent occurs where it is desirable to measure flow velocity with regard to a known corrdinate system which is not defined or related to the North magnetic pole. Thus, for instance, a user might be interested in the flow velocity at the mouth of a channel or river, in which case it would be desirable to measure flow perpendicular to the longitudinal extend of the channel or river. In this case, determining flow velocity with regard to the directions of North and East would only introduce the additional complexity of further data reduction to obtain flow velocity in the desired direction.
A further example of situations in which the 3,641,816 apparatus would not be usable, is the situation of measuring flow velocity in areas where the earth's magnetic field may be masked by artificially induced fields. In this case, of course, the magnetometers would not be able to sense the earth's magnetic field and thus the 3,641,816 apparatus would not be usable.
In view of the foregoing, it is an object of the present invention to provide apparatus for conditioning the output of an electromagnetic water current meter to provide signals representative of orthogonal flow velocity components with reference to a known coordinate system, which known coordinate system is not defined by the earth's North magnetic pole. It is another object of the present invention to provide such a device in which the knwon coordinate system can be located at will in any desired direction. Still other and further objects of the invention will be apparent from a reading of this application.